1. Field of the Invention
The present invention relates to a ball screw assembly, and more particularly to a screw shaft, wherein a helical groove is formed in the outer surface of the screw shaft without extending to both ends thereof, and an escape portion in the form of a slot is connected to one end of the helical groove of the screw shaft, so that the tongue portion of the cassette can be rotated in the helical groove of the shaft along with the axial movement of the nut.
2. Description of the Prior Art
The rolling elements in a ball screw usually circulate in three ways: inner circulation, outer circulation, and end cap circulation. As shown in FIG. 1, which is a stereographic view of a conventional ball screw assembly whose balls 11 circulate through the axial hole of the nut 20, wherein a cassette 10 is fixed in a recess 201 formed in an end surface of the nut 20. The cassette 10 is defined with a return path 101 for the balls 11, and the cassette 10 is pushed in the axial direction of the nut 20 until the axial positioning portion 102 on the cassette 10 is positioned in the recess 201 of the nut 20, thus limiting the axial motion of the cassette 10. And then a screw 30 is screwed through the positioning hole 202 of the nut 20 into a locking hole 103 of the cassette 10, thus restricting the radial motion of the cassette 10. According to this assembly method, it must define a helical groove 402 in a shoulder portion 401 of the screw shaft 40 for facilitating the assembly of the nut 20. Hence, this assembly method is unsuitable for the helical groove defined directly in the screw shaft 40.
However, the helical groove 402 must extend to the end surface of the shoulder portion 401 for facilitating the assembly of the nut 20, and this will form a notch 404 in the abutting surface 403 of the shoulder portion 401. The notch 404 makes the screw shaft 40 impractical because of the following factors:
First, the helical groove 402 of the screw shaft 40 left a notch 404 in the abutting surface 403 of the shoulder portion 401, as a result, the integrality of the diameter of the abutting surface 403 is damaged (the outer periphery of the abutting surface 403 is not round). When both ends of the screw shaft 40 are mounted on bearing seats of a machine (not shown), and the motor of the machine starts to rotate the screw shaft 40, the rotation inertia will effect an axial load on both ends of the screw shaft 40. And due to the integrality of the diameter of the abutting surface 403 is damaged, the axial load cannot be evenly distributed on the both ends of the screw shaft 40. This will generate great pressure against the end surface (not shown) of the bearing seats, causing damage to the bearing seats, as a result of that, the screw shaft 40 will be deformed during rotation, adversely affecting the circulation of the rolling elements, but will reduce the service life of the screw shaft. Therefore, it is uneconomical.
Second, when both ends of the screw shaft 40 are mounted on bearing seats of a machine, the screw shaft 40 is then inserted through the nut 20 and filling balls, the ball screw is assembled by hand. The ball screw probably needs to be subjected to adjustment process (the respective components of the ball screw should be adjusted) during assembly, because of the adjustment process, the balls may drop out of the ball screw when the nut 20 is disengaged from the screw shaft 40 or when the nut 20 moves out of travel.
Hence, with the popularization of the linear transmission application, the demand of solving the aforementioned problems is becoming more and more urgent, and manufacturing an easily assembled and low cost ball screw assembly has become the most important selling point that the manufactures are striving for.